The invention generally relates to a combined heat and power fuel cell system and associated methods of operation.
A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. For example, one type of fuel cell includes a polymer electrolyte membrane (PEM), often called a proton exchange membrane, that permits only protons to pass between an anode and a cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel) is reacted to produce protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the protons to form water. The anodic and cathodic reactions are described by the following equations:
H2xe2x86x922H++2exe2x88x92 at the anode of the cell, and
O2+4H++4exe2x88x92xe2x86x922H2O at the cathode of the cell.
A typical fuel cell has a terminal voltage of up to about one volt DC. For purposes of producing much larger voltages, multiple fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide more power.
The fuel cell stack may include flow field plates (graphite composite or metal plates, as examples) that are stacked one on top of the other. The plates may include various surface flow field channels and orifices to, as examples, route the reactants and products through the fuel cell stack. A PEM is sandwiched between each anode and cathode flow field plate. Electrically conductive gas diffusion layers (GDLs) may be located on each side of each PEM to act as a gas diffusion media and in some cases to provide a support for the fuel cell catalysts. In this manner, reactant gases from each side of the PEM may pass along the flow field channels and diffuse through the GDLs to reach the PEM. The PEM and its adjacent pair of catalyst layers are often referred to as a membrane electrode assembly (MEA). An MEA sandwiched by adjacent GDL layers is often referred to as a membrane electrode unit (MEU).
A fuel cell system may include a fuel processor that converts a hydrocarbon (natural gas or propane, as examples) into a fuel flow for the fuel cell stack. For a given output power of the fuel cell stack, the fuel flow to the stack must satisfy the appropriate stoichiometric ratios governed by the equations listed above. Thus, a controller of the fuel cell system may monitor the output power of the stack and based on the monitored output power, estimate the fuel flow to satisfy the appropriate stoichiometric ratios. In this manner, the controller regulates the fuel processor to produce this flow, and in response to the controller detecting a change in the output power, the controller estimates a new rate of fuel flow and controls the fuel processor accordingly.
A fuel cell system may include a fuel processor that converts a hydrocarbon (natural gas or propane, as examples) into a fuel flow for the fuel cell stack. For a given output power of the fuel cell stack, the fuel flow to the stack must satisfy the appropriate stoichiometric ratios governed by the equations listed above. The amount of a reactant supplied may be referred to in terms of xe2x80x9cstoichxe2x80x9d. For example, for a given electrical load on a fuel cell, one stoich of hydrogen and one stoich of air would refer to the minimum amount of each reactant theoretically required to produce enough electrons to satisfy the load (assuming all of the reactants will react). However, in some cases, not all of the hydrogen or air supplied will actually react, so that it may be necessary to provide excess fuel and air stoichiometry so that the amount actually reacted will be appropriate to satisfy a given power demand.
Hydrogen that is not reacted in the fuel cell may be vented to the atmosphere with the fuel cell exhaust, and in some cases may be oxidized before it is vented. Such exhaust may also contain small amounts of hydrocarbons that xe2x80x9cslipxe2x80x9d through the fuel processor without being reacted. Substantial heat may be generated as these exhaust components are oxidized, for example by mixing them with air and passing them through a platinum-coated ceramic monolith similar to an automotive catalytic converter.
The fuel cell system may provide power to a load, such as a load that is formed from residential appliances and electrical devices that may be selectively turned on and off to vary the power that is demanded by the load. Thus, the load may not be constant, but rather the power that is consumed by the load may vary over time and abruptly change in steps. For example, if the fuel cell system provides power to a house, different appliances/electrical devices of the house may be turned on and off at different times to cause the load to vary in a stepwise fashion over time.
There is a continuing need for systems and algorithms to achieve objectives including the foregoing in a robust and cost effective manner.
The invention provides a combined heat and power fuel cell system and associated methods of operation. Such systems are commonly referred to as cogeneration systems. In general, the system and methods of the invention relate to operation of a fuel cell system among various modes and configurations to balance heat and power demand signals. The fuel cell system is coupled to both a power sink and a heat sink. A controller is adapted to coordinate response to data signals from the power sink and the heat sink. As examples, such data signals from the heat sink may include a temperature indication or a heat demand signal (such as from a thermostat), and such data signals from the power sink may include a voltage or current measurement, an electrical power demand signal, or an electrical load.
In one aspect, a fuel cell system is provided that includes a fuel cell, a fuel supply, an oxidant supply, a power demand sensor, a heat demand sensor, and a controller. The fuel cell is adapted to receive a fuel flow from the fuel supply, and an oxidant flow from the oxidant supply. The controller is connected to each of the fuel supply, oxidant supply, power demand sensor, and heat demand sensor. The controller is further adapted to receive a power demand signal from the power demand sensor and a heat demand signal from the heat demand sensor.
In a first state, the controller is configured to reduce at least one of the fuel flow and oxidant flow when there is no heat demand signal and no power demand signal. In a second state, the controller is configured to increase at least one of the fuel flow and oxidant flow when there is no heat demand signal and there is a power demand signal. In a third state, the controller is configured to increase at least one of the fuel flow and oxidant flow when there is no power demand signal and there is a heat demand signal. In a fourth state, the controller is configured to increase at least one of the fuel flow and oxidant flow when there is a power demand and a heat demand signal.
In some embodiments, the power demand sensor is a fuel cell voltage sensor that produces a power demand signal when a voltage of the fuel cell falls below a predetermined level. The power demand sensor can also be a fuel cell current sensor that produces a power demand signal when an output current of the fuel cell exceeds a predetermined level. The power demand sensor can also include a fuel cell output current sensor an electrical load sensor, wherein the power demand sensor produces a power demand signal when an electrical load on the fuel cell exceeds an output current of the fuel cell. It will be appreciated that the electrical load on the fuel cell can include a parasitic system electrical load and an application electrical load. For example, the parasitic load can refer to internal components such as pumps and blowers that are powered by the fuel cell. The application load can refer to a residential appliance, as an example.
The system can further include a coolant circuit and a heat sink, wherein the coolant circuit is adapted to transfer heat from the fuel cell to the heat sink. As an example, the heat demand sensor can be a temperature sensor that produces a heat demand signal when a temperature of the heat sink is below a predetermined level.
In one embodiment, the system can include a heat sink, a coolant circuit, and an oxidizer adapted to oxidize an exhaust gas of the fuel cell. The coolant circuit is configured to transfer heat from the fuel cell to the heat sink, and the heat demand sensor is a temperature sensor that produces a heat demand signal when a temperature of the heat sink is below a predetermined level. In another embodiment, the coolant circuit is adapted to transfer heat from the fuel cell to the heat sink, and a radiator is provided to remove heat from the coolant circuit. The radiator can include a fan connected to the controller, where the controller is configured to reduce an output of the fan when there is a heat demand signal. The controller is further configured to increase an output of the fan when there is no heat demand signal.
In another embodiment, the coolant circuit further includes a bypass valve and a radiator bypass circuit. The valve is connected to the controller, and the controller is adapted to actuate the valve to divert a coolant flow from the radiator to the radiator bypass circuit when there is a heat demand signal. The controller is further adapted to actuate the valve to divert the coolant flow from the radiator bypass circuit to the radiator when there is no heat demand signal.
The system can also include a fuel bypass circuit associated with the valve. In such a system, the valve is connected to the controller, and the fuel bypass circuit is adapted to divert a portion of the fuel flow from an inlet of the fuel cell to the oxidizer. The controller is configured to actuate the valve to divert the portion of fuel flow from the fuel cell inlet to the oxidizer when there is a heat demand signal. The controller is further adapted to actuate the valve to divert the portion of fuel flow from the fuel cell inlet to the oxidizer when there is no heat demand signal. As an example, the controller can include a computer usable medium (e.g., memory) having computer readable code embodied thereon (e.g., firmware or software). Preferably, the controller is also programmable.
Embodiments may further include a hydrogen separator, such as electrochemical hydrogen separator. The hydrogen separator is adapted to receive the fuel flow from the fuel processor and separate hydrogen from the fuel flow into a reservoir when the hydrogen separator is activated. The controller is configured to activate the hydrogen separator when there is no power demand signal and there is a heat demand signal.
As an example, the hydrogen separator can include a membrane electrode assembly having an anode side and a cathode side. It is well known in the art that placing an electric potential across an electrochemical cell, such as a fuel cell, having no electrical load (as opposed to merely placing an electric load on the fuel cell as in the case of normal operation) will result in hydrogen being electrochemically xe2x80x9cpumpedxe2x80x9d from fuel (e.g., reformate) in the anode to the cathode. This process proceeds essentially according to the same reactions at the anode and cathode of the fuel cell as in normal operation.
For example, such a cell can be placed along the flow path of the reformate being fed from the fuel processor to the fuel cell. When there is a heat demand, but no power demand, the controller reacts enough fuel in the fuel cell to produce the desired amount of heat. The excess power is sunk to the hydrogen separator to pressurize a hydrogen tank (e.g., at about two atmospheres), which will contain essentially pure hydrogen. The hydrogen tank reservoir can include a valve connected to the controller and associated with a conduit to the fuel cell such that the controller can selectively open the valve to supply hydrogen to the fuel cell (e.g., in response to a sudden load increase).
The hydrogen separator can be a PEM fuel cell (e.g., a PEM sandwiched on either side by a platinum based catalyst layer). The anode side is in fluid connection with the fuel flow from the fuel processor. The anode side and cathode side of the membrane electrode assembly each have an electrical connector (e.g., a wire connected to the each of the anode and cathode flow field plates. A power source is connected to the anode and cathode electrical connectors of the membrane electrode assembly and provides an electric potential across the connectors when the separator is in an active state. Similarly, the controller can remove the potential to put the separator in an inactive state. While the separator is in the inactive state, the reformate simply passes by it on the way to the fuel cell without effect. In some embodiments, the separator can also be used, as can the hydrogen reservoir supply to the fuel cell, when there is a power demand.
Advantages and other features of the invention will become apparent from the following description, drawings and claims.